GeezerVison

Summary

Adds environment variable equivalents for four flags that are commonly needed when running linuxkit on CI/CD self-hosted runners with private registry mirrors, so callers (e.g. make in downstream projects) don't need to be modified to pass these flags explicitly.

Priority: CLI flag > env var > built-in default (consistent with existing LINUXKIT_CACHE, LINUXKIT_BUILDER_NAME, LINUXKIT_BUILDERS).

LINUXKIT_MIRROR accepts the same format as --mirror ([registry=]url), as a space- or comma-separated list. Env var entries are prepended to CLI-provided entries so that the final SetProxy call for a given registry key comes from the CLI flag, giving it precedence.

LINUXKIT_BUILDER_IMAGE and LINUXKIT_BUILDER_CONFIG use the existing flagOverEnvVarOverDefaultString helper (same pattern as LINUXKIT_BUILDER_NAME).

Test plan

• go build ./... passes
• go test -mod=vendor ./... passes (all existing tests green)
• Set LINUXKIT_MIRROR=docker.io=http://mymirror and verify linuxkit routes pulls through the mirror
• Set LINUXKIT_PKG_ORG=myorg and verify linuxkit pkg show-tag reflects the org override
• Set LINUXKIT_BUILDER_IMAGE and verify the builder container uses the specified image
• Verify CLI flags override the env vars when both are set

🤖 Generated with Claude Code